Cermet materials such as cemented tungsten carbide (WC--Co) are well known for their mechanical properties of hardness, toughness and wear resistance, making them a popular material of choice for use in such industrial applications as cutting tools for machining, mining and drilling where its mechanical properties are highly desired. Cemented tungsten carbide, because of its desired properties, has been a dominant material used in such applications as cutting tool surfaces, hard facing, wear component and roller cone rock bit inserts, and cutting inserts in roller cone rock bits, and as the substrate body for drag bit shear cutters. The mechanical properties associated with cemented tungsten carbide and other cermet material, especially the unique combination of hardness, toughness and wear resistance, make this class of materials more desirable than either metal or ceramic materials alone.
For conventional cemented tungsten carbide, the mechanical property of fracture toughness is inversely proportional to hardness, and wear resistance is proportional to hardness. Although the fracture toughness of cemented tungsten carbide has been somewhat improved over the years, it is still a limiting factor in demanding industrial applications such as high penetration drilling, where cemented tungsten carbide inserts often exhibit gross brittle fracture that can lead to catastrophic failure. Traditional metallurgical methods for enhancing fracture toughness, such as grain size refinement, cobalt content optimization, and strengthening agents, have been substantially exhausted with respect to conventional cemented tungsten carbide.
The mechanical properties of commercial grade cemented tungsten carbide can be varied within a particular envelope by adjusting the cobalt metal content and the tungsten carbide grain sizes. For example, the Rockwell A hardness of cemented tungsten carbide can be varied from about 85 to 94, and the fracture toughness can be varied from about 8 to 19 Mpam.sup.-2. Applications of cemented tungsten carbide are limited to this envelope.
Polycrystalline diamond is another type of material that is known to have desirable properties of hardness, and wear resistance, making it especially suitable for those demanding applications described above where high wear resistance is desired . However, this material also suffers from the same problem as cemented tungsten carbide, in that it also displays properties of low fracture toughness that can result in gross brittle failure during usage.
It is, therefore, desirable that a composite construction be developed that has improved properties of fracture toughness, when compared to conventional cermet materials such as cemented tungsten carbide materials, and when compared to conventional materials formed from polycrystalline diamond or cubic boron nitride. It is desirable that such composite construction have such improved fracture toughness without sacrificing other desirable properties of wear resistance and hardness associated with conventional cemented tungsten carbide, polycrystalline diamond, and polycrystalline cubic boron nitride materials. It is desired that such composite constructions be adapted for use in such applications as roller cone bits, hammer bits, drag bits and other mining, construction and machine applications where properties of improved fracture toughness is desired.